Many red and some white wine varieties are matured in oak barrels, according to the wine variety, quality and desired character.
The general aim of wine aging in Oak barrels, is to develop a desired aged wine bouquet and character, by:                Controlled and slow oxidation of various wine substances especially the phenolic compounds. This slow oxidation, followed by polymerization, results from limited and gradual exposure to air oxygen diffusing through the walls of the barrel over months.        Extraction of Oak flavour components, including Oak phenolics to enhance and expand the wine's complexity.        
However Oak barrels are expensive to fashion, cumbersome to store and handle, often inconsistent in wood properties, subject to leakage and limited in useful life.
Oak barrels also require a voluminous storage facility with a controlled temperature and humdidity environment to minimize evaporative loss due to transpiration through their porous walls.
Accordingly there have been many attempts to replicate the effects of barrel aging on wine, without the actual use of oak barrels. For example:                The addition of Oak sawdust, chips or staves into metal bulk storage tanks, to expose the wine to Oak characters. However this only achieves the Oak flavour extraction function of barrel aging, not the oxidative one. This can be mitigated to some extent by regularly “pumping over” the wine in the tank, in order to re-aerate it. However this often introduces too much dissolved oxygen all at once, allowing aerobic bacteria such as Acetobacter to become established and begin oxidizing the ethanol in the wine into acetaldehyde, ethyl acetate and ultimately acetic acid, with consequent loss of wine quality. These oxidation products are termed “volatile acidity” in the wine industry.        The development of composite containers made of metal (stainless steel) and fitted with flat Oak panels or ends which can be more easily fashioned and which may be reversed to expose fresh Oak to the wine contents. Such containers usually don't have the optimum ratio of surface area of Oak to volume of wine contained and are usually inadequate both in the extractive and oxidative functions.        
More recent developments include the use of “micro-oxygenation” wherein air or pure oxygen is introduced directly into bulk-tanked wine with added Oak chips or planks, by generating very fine bubbles through the wine by means of a micro-porous (ie. sintered) dispersion element on the end of a submerged gas line or lines.
This is a difficult process to adequately control and can possibly lead to excess levels of dissolved oxygen at the bubble interfaces, again promoting ethanol oxidation and other degradative side-effects through fast oxidation of wine. The equipment required to contain and feed fixed volumes of oxygen or to accurately meter a continuous flow over set time periods at very low flow rates, is normally expensive and difficult to operate. That cost also means the resource has to be shared over a number of tanks, which militates against long, slow maturation times in individual tanks.
Recent studies such as reported in “Gaseous Exchange in Wine Stored in Barrels”, Moutounet, Mazauric et al, J. Sci. Tech. Tonnellerie, 1998 (herein incorporated by reference) have explained the mechanism by which barrels add oxygen to wine and promote “correct” maturation. The oak walls of wine barrels act as semi-permeable “membranes”, allowing oxygen gas in the atmosphere outside the barrel to permeate through the barrel walls and diffuse into the wine. This osmotic exchange is driven by the partial pressure difference of the atmospheric oxygen outside the barrel (0.18 atm) and the effectively zero partial pressure of oxygen inside. This absence of oxygen in the barrel arises because wine continuously consumes all available oxygen in the slow-oxidation reactions that occur in the reductive environment that exists when the rate of oxygen addition occurs at very low rates over long periods, due to slow diffusion.
Furthermore, it has been shown that it is this slow permeation (diffusion) of atmospheric oxygen that contributes most to “barrel softening or maturation”. Moutounet et. al. (referred to above) showed that a typical new oak barrique allows O2 permeation through its walls, in the range of 20-30 mg/l.yr. Kelly and Wollan report an estimated “highest diffusion” (ie. permeation) rate” into a typical barrique as 2.2 ml O2/liter wine/month or 26.4 ml/l/yr (34.6 mg/l./gr) in their paper “Micro-oxygenation of Wine in Barrels”, Wine Network Technology, www.winenet.com.au, incorporating International Patent Application PCT/AU02/01250 (both herein incorporated by reference). As used hereinafter in the application and claims, the phrase “oak cask maturation” is defined as maturation that takes place in oak casks at the levels of oxygen permeation normally associated with maturation in oak casks.
Any free surface arising from head-space in a wine storage vessel is undesirable, if it contains oxygen. At that surface the levels of dissolved oxygen increase in concentration, to near saturated (9 ppm at 20 Celsius and 1 atm). In this oxygen rich surface layer aerobic bacteria will propagate, generating volatile acidity and acetaldehyde at a rate dependent usually on the surface area of the free surface and the oxygen content of the surface gas. The lesser the free surface area, the longer that wine can be safely kept in bulk storage.